1. Field of the Invention
The present invention relates to a lighting device, for example, a lighting device which emits light by luminescence.
2. Description of the Related Art
In recent years, light-emitting devices including organic electroluminescence (hereinafter also referred to as EL) elements as self-luminous elements have been actively researched. In the basic structure of an organic EL element, a layer including a light-emitting organic compound (hereinafter referred to as a light-emitting layer) is provided between a pair of electrodes. By voltage application to this element, light emission from the light-emitting organic compound can be obtained.
Since an organic EL element can be formed in a film form, planar light emission can be easily obtained. This is a feature which is difficult to obtain in point light sources typified by an incandescent lamp, or line light sources typified by a fluorescent lamp and the like. Specifically, since an organic EL element is a surface light source, lighting with less glare of the light source which enters the field of view can be provided.
In an organic EL element, a plurality of light-emitting layers are provided and light is emitted from each light-emitting layer, whereby light emission in which plural types of light emission are mixed can be obtained. For example, light-emitting layers which emit light of respective colors of red, green, and blue, or light-emitting layers which emit light of complementary colors are overlapped with each other (or stacked), whereby an organic EL element which emits white light can be obtained.
A lighting device including an organic EL element, which makes use of these characteristics, is disclosed (see Patent Document 1).
As a technique which is often compared with an organic EL element, an LED (also referred to as a light-emitting diode) can be given. An LED is a point light source having high directivity. In recent years, an LED has been showing rapid growth in market share for lighting owing to a significant increase in a lifetime. Some LED lamps have a lifetime of more than a hundred thousand hours and such LED lamps are employed for traffic lights, streetlights, and the like to make use of the characteristic of resistance for long-term use. Accordingly, the frequency of replacement of a lamp is significantly reduced, which leads to drastic retrenchment, conservation of resources, and the like. Note that an LED in this specification refers to, in particular, a point light emitting element among semiconductor elements which emit light when forward voltage is applied.
There are a variety of methods of manufacturing white lighting using an LED. As a device structure which has been mainly considered, a structure in which a blue LED and a phosphor emitting yellow light or the like are combined can be given. In this structure, the phosphor is provided around the blue LED. A white light source is realized by a mixture of light: light obtained when the phosphor absorbs light from the blue LED and performs conversion, and blue light which passes through the phosphor without being absorbed (see Patent Document 2).